This relates generally to reflective polarizers which transmit one polarization and reflect the other polarization. More particularly, this application relates to reflective polarizers having an extended red band edge in the off-axis transmission spectrum to reduce off axis color hue, or to select a particular hue.
One important use for reflective polarizers is in a light-recycling mode to provide brightness increase (gain) in liquid crystal display (LCD) applications. In these applications, the reflective polarizer is used in conjunction with a lightrecycling cavity, a light source, and a liquid crystal panel. Maximum luminance is achieved when the reflective polarizer is used in a brightness enhancement mode, such that light of the reflected polarization is xe2x80x9crecycledxe2x80x9d into the transmitted polarization by the reflective polarizer in combination with the light recycling cavity. Examples of such light recycling systems are described in copending and commonly assigned U.S. patent application Ser. No. 08/402,134, now U.S. Pat. No. 5,882,774, which is incorporated herein by reference.
The off axis color (OAC) problem with multilayer-type reflective polarizers is described in copending and commonly assigned U.S. patent application Ser. No. 08/402,041, which is incorporated herein by reference. OAC is also a problem with other types of reflective polarizers, such as cholesteric reflective polarizers. The multilayer reflecting polarizers described in the above mentioned U.S. patent application Ser. No. 08/402,041 show a very red or yellow OAC. In other words, the OAC has a distinct reddish or yellowish xe2x80x9chuexe2x80x9d. This is because the red band edge of the off axis transmission spectrum is shifted toward the blue. As a result, almost all p-polarized red light (600 nm and greater) is transmitted at off normal incidence, but some of the blue and green is reflected, giving the light at off normal incidence a definite reddish appearance. This hue is very objectionable in some applications, such as in the backlit LCD systems, where color control is highly important.
To overcome the drawbacks in the art described above, and to provide various other advantages will become apparent upon reading and understanding the present specification, the present invention provides a reflective polarizer with an extended red band edge to reduce off axis color (OAC). The reflective polarizer has a OAC red band edge of at least 600 nm at a 45 degree angle of incidence, and has an average transmission from 400-800 nm of less than 20% for light polarized parallel to the extinction axis at normal incidence. This results in a more color balanced system in which the color bands across the visible spectrum are more evenly reflected, thus controlling the hue of the OAC.